Wellhead apparatus



Oct. 16, 1956 R. P. VINCENT WELLHEAD APPARATUS Filed Aug. 20, 1953 m 1 A /A x 4 5 d 2 if Y United States Patent M WELLHEAD APPARATUS Renic P. Vincent, Tulsa, Okla, assignor to Stanolind Oil and Gas Company, Tulsa, Okla., a corporation of Delaware Application August 20, 1953, Serial No. 375,341

6 Claims. (Cl. 103-52) This invention relates to an apparatus for reciprocating a temperature-responsive free piston in a well tubing. More particularly, thi invention relates to a wellhead apparatus for cooling a temperature-controlled free piston operating in a well tubing to produce a well or to remove paraffin from the tubing walls.

In my copending application, S. N. 197,048 filed November 22, 1950, now U. S. Patent 2,704,980, a thermallyactuated free type piston for producing wells is described and claimed. The free piston or plunger, as it is sometimes called, is adapted to reciprocate in a vertical tubing from a bottom stop in the tubing to the wellhead. Free pistons are caused to reciprocate in the tubing by mechanical, pressure, and thermal control means. That is, by one of these characteristics of the surrounding fluid, the control mechanism in the free piston is shifted so that when the free piston reaches the end of either the power stroke or the return stroke, its motion is automatically reversed. When, for example, a thermallyactuated piston reaches the bottom of a well or a bottom stop in the well and it is submerged in the warm well fluids, the control mechanism is actuated to close a fluid bypass in the piston so that the free piston is raised by the well fluids or by a lifting gas to the surface. At the surface after the well fluids have been displaced from the tubing into the flow line, the control valve is again shifted automatically by a temperature change so that the free piston falls by its own weight through the tubing to the bottom position where a new cycle is commenced.

In the temperature-responsive or thermally-actuated free piston, as it is described in said copending application, the free piston is lowered into the well with the packing element between the body of the free piston and the wall of the tubing contracted so that the free piston falls freely to the bottom stop. At thebottom position in the tubing the packing element is automatically expanded by the expansion of an associated chamber filled with a liquid having a high thermal coeflicient of expansion. That is, due to the increased temperature at the bottom of the well relative to the surface temperature, the liquid in the enclosed chamber expands causing the packer or paraffin scraper to expand diametrically and fill the tubing so that any upward flow of fluid in the tubing tends to lift the free piston to the surface. At the surface, the temperature being lower than at bottom hole conditions, the thermally-sensitive liquid within the free piston contracts causing the packing element to contract diametrically and allow the free pistons to fall through the tubing.

As shown in Petroleum Development and Technology, Petroleum Division, 1947, page 44, the temperature gradients in oil Wells vary somewhat in oil-producing provinces. A gradient of 0.4 per 100 feet change in elevation is common, for example, in southeastern New Mexico, and a temperature gradient of as much as 2 or more per 100 feet of elevation difference is not uncommon in the Gulf Coast region. I have now found that due to these extreme variations in temperature gradients in wells, par- 2,766,694 Patented Oct. 16, 1956 ticularly the very low gradients in some areas, and due to the wide variations in surface temperatures between night and day and between seasons and locations, in some areas a temperature-sensitive free piston cannot be set to operate over extended periods of time without some periodic adjustments. For example, in some cases the wellhead temperature due to high surface temperature conditions may sometimes be above the bottom hole temperature or the temperature at the elevation of the bottom stop in the tubing. Also, while there is practically universally a temperature gradient between the bottom and top of the well with the cooler temperature which might be relied upon to contract the packing element close to the surface, it is particularly desirable, especially in the case of a paraffin scraper, that the free piston rise all the way through the tubing to the flow line connection and not stop or reverse direction at the coolest subsurface temperature. I have found, therefore, that to avoid an unreasonably critical setting of the control mechanism for the temperature-actuated free type piston that artificial cooling of the wellhead is often desirable.

It is, therefore, an object of this invention to provide an improved method and wellhead apparatus for reciprocating a thermally-sensitive free piston in a well. A more specific object of this invention is to provide artificial cooling for a thermally-sensitive free piston at its upper position in the well tubing. A still more specific object of this invention is to provide means for cooling a temperature-sensitive free piston at its extreme upper position in a well tubing by expanding the compressible fluids in the tubing and utilizing the energy of expansion for cooling the free piston. These and other objects of this invention will become apparent from the following description in which:

Figure 1 is a cross-sectional view of a wellhead connection showing apparatus adapted to cool the free piston in its uppermost position by expansion of the Well fluids; and

Figure 2 is a cross-sectional view of a diagrammatic reproduction of one embodiment of an improved gas lift plunger.

This invention thus in brief comprises a free piston pumping system in which the free piston is artificially cooled at the endof one stroke, and more particularly it comprises a wellhead apparatus used to refrigerate the free piston. In its broader aspects, artificial cooling of the free piston includes all means of cooling other than that provided by nature. In the preferred embodiment, it includes the absorption of heat from a compressed gas which is expanded. In this embodiment the volatile fluids which lift the free piston are expanded through a choke and the expanded and cooled gases are then passed in indirect heat exchange with the temperaturesensitive element of the free piston to contract the paclo ing element and allow the free piston to fall through the well fluids in the tubing to the bottom of the well.

Referring now specifically to Figure l, the upper end of a tubing string 3 which may be set in a tubing head 4 extends into a well 5 to the fluid level. This tubing is used to produce gas or a liquid such as oil or water from the well. The special cooling apparatus is connected between the well tubing and the flow line. It consists first of a tubing extension or inside tube 6 which has an internal diameter substantially equal to the internal diam eter of tubing 3 and which is attached to the upper end of the tubing by the collar 7. An orifice fitting 8 is inserted in a wall of inside tube 6. This orifice fitting may be removable and replaceable with chokes 9 of dif ferent sizes. A casing or outside tube 10 having an inside diameter substantially greater than the outside diameter of inside tube 6 surrounds the inside tube 6 leaving an annular chamber 11 through which well fluids flow from choke 9 to flow line outlet 12 and thence to storage or the like. The annular chamber 11 is closed at the upper end by head Band at the lower endby head 14. In some cases, it is considered desirable to have an adjustable choke so that the flow rate of the well can be controlled or so that the temperature of the gases issuing from choke 9 can be varied without interrupting the production of the well. In such cases, a threaded outlet 15 is provided in and near the bottom of outside tube with the opening coaxial with choke 9. This outlet not only provides access to the choke so that the chokes size may be changed, but it also provides means for attaching an adjustable needle valve mechanism 16 by which the pressure drop through the choke and the temperature in cooling chamber 11 can be controlled. The inside tube 6 extends above choke 9 any desirable distance, typically between 3 and about 10 feet, and has a cap 17 closing the upper end to form a damping chamber 18 which cushions the free piston 19 when it rises at high velocity through the tubing 3.

In operation the free piston 19 is dropped into tubing by removing cap 17. The temperature of the free piston being lower than bottom hole temperature, the packing or scraping element 21 is in a contracted position so that the free piston falls freely through the tubing to a bottom stop (not shown). In case the free piston is used as a pump for producing the well, the bottom stop, as previously indicated, is preferably located in the tubing string at an elevation below the Working fluid level. Temperature-sensitive free pistons are particularly adaptable for scraping a paraflin deposit from the inside tubing wall in a flowing or gas-lift well since the rate of reciprocation or the number of trips per day can be adjusted to a very low frequency. For example, in a deep well by providing a thermal element with a substantial time lag, the free piston can be made to produce as low as one cycle per day. On the other hand, its frequency can be increased so that a cycle can be made every few minutes depending upon the depth and flow rate of the well, the sensitivity of the thermally-controlled actuating mechanism, etc. In a typical well, where paraflin deposition in the tubing is a particular problem, from 2 to about 10 cycles or more per day are considered desirable. As is well known, paraffin deposition on the inside tubing surface may occur at only short intervals along the tubing, usually near the surface. Accordingly, it may be desirable that the free piston have a stroke of only a few hundred or perhaps a thousand feet in a much deeper well. The length of the stroke is normally controlled by placement of the bottom stop. It can also be controlled by adjusting the temperature-sensitive actuating element within the free piston so that the packer is expanded before the free piston reaches the bottom of the well even if no bottom stop is used. A relatively large temperature differential between the well head and bottom hole temperature is desirable in such cases. The difference between the temperature at the tubing stop and the well head must be suflicient in any case, typically between about 10 and about 25 F., to operate the temperaturesensitive control means. It is also important that means be provided to lower the temperature of the free piston at the surface by from about 2 to about 10 F. or more below the normal wellhead temperature of the well fluids.

After the free piston falls through the Well and is stopped by the bottom stop in a flowing well, it remains at the bottom stop until the temperature of the well fluids flowing past the piston warms the temperature-sensitive element located in the upper section 22 of the free piston to the pre-set temperature at which packer 21 is expanded and packs off the tubing. After the packer has thus been expanded, the well fluids lift the free piston through the tubing until the packet 21 is located above choke 9 leaving the temperaturesensitive element in section 22 of the free piston positioned Within the cooling chamber 11. inasmuch as the free piston provides a moveable barrier in the 4 tubing, the well fluids below the free piston are divided leaving free gas immediately below the piston. This free gas, after displacing the free piston 19 above choke 9, escapes through the choke and is cooled by adiabatic expansion. This cooled gas then circulates up through chamber 11 cooling that chamber and by indirect heat exchange cooling the temperature-sensitive element located in section 22 of the free piston. The well fluids ahead of free piston 19 which were also displaced through orifice 9 and chamber 11 are displaced by this expanded gas through flow line outlet 12 to storage. This cooling of the temperature-sensitive element in the free piston causes the packer to be contracted so that the free piston falls back down the tubing bypassing well fluids which are rising therein, and another cycle is commenced.

Turning now to a detailed description of the construction and operation of a typical free piston which is described and claimed in the above-mentioned copending application, Figure 2 shows the elements of one type of such apparatus which is adapted to be reciprocated in a vertical tubing by the difference in temperatures at the bottom and top of the tubing. In this embodiment, an expansible packing or scraping element 21 is expanded by the movement of a piston 41. Piston 41 operates in a cylinder 42 having a cylinder head 43. This cylinder, piston and cylinder head define a variable-volume chamber 44 which is filled with a liquid. The packer is confined between piston 41 and a circular plate 45. This plate and the cylinder head 43 are rigidly connected by a rod 46, so that the distance therebetween remains substantially constant. Packer 21 and piston 41 are, preferably, coaxial with this rod. The piston is in sealing engagement with the rod by sealing ring 47 and with cylinder 42 by sealing ring 48.

The resilient scraping element or packer 21, which may be constructed of synthetic rubber or any other resilient material which will not deteriorate in the well, provides the sealing element which prevents the lifting gas from by-passing the free piston on the power stroke. This packer, which is loosely fitted on rod 46, has a nominal outside diameter substantially less than the diameter of the tubing in which it is to operate. For example, a packer between about 1 /2 and 2 inches in diameter is desirably employed in 2 /i-inch tubing. The length of the packer may be varied over a substantial range, but I prefer that the length be greater than the diameter. In many cases, the length is desirably from 2 to 5 times or more greater than the diameter.

As mentioned above, chamber 44 is filled with a substantially non-compressible fluid. This fluid also, preferably, has a high coefficient of cubical expansion. Water,

which has a coeflicient of cubical expansion of 0.000115 inch per degree F., can be used satisfactorily; but the size of the apparatus, including particularly the volume of the variable volume chamber, can be decreased by the use of other liquids having higher coeflicients of cubical expansionfor example, petroleum oils, having cocflicients in the order of 0.0005, ether, having a coefficient of cubical expansion of 0.00092, and methanol, having a coefficient of cubical expansion of 0.0008, may be used to advantage in many cases. Other incompressible fluids, such as rubber, which h ve a high coeflicient of cubical expansion and behave like a liquid may, in some cases, be substituted for the above-mentioned liquids. For example, if methanol is used as the expanding liquid in the variable volume cell, a length of cell of 6 inches produces a ,di-inch change in the length of the packer 21 for F. change in cell temperature. This change in packer length is sufiicient to expand the maximum packer diameter approximately from 2.5 to 2.61 inches, when using a packer 6 inches long, 2.5 inches in outside diameter and 1.75 inches inside diameter. If petroleum oil is used, the minimum cell length for such expansion is 10 inches. Using water as the expansion medium, a cell length of about 4 feet is required. Using methanol, a cell 19 inches long produces in this same packer a diameter change from 2.5 to 2.7 inches for 100 F. temperature increase.

In operation, the plunger is assembled as indicated, the variable volume chamber 44 being filled with any of the above-mentioned liquids. Gases are, as completely as possible, displaced from this chamber. In some cases, however, it may be desirable to inject into the chamber a small amount of an inert gas, such as nitrogen. This gas provides a slight amount of cushioning which permits the diameter of packer 21 to be varied over a small range as it passes through the tubing which has minor variations in diameter. The volume of liquid placed in chamber 44 is, obviously, adjusted so that, when bottom-hole temperature is reached, piston 41 will not be forced out of cylinder 42. Under normal conditions, the chamber may be filled through the central bore 49 when the temperature of the liquid is at the temperature of the lifting gas at the well head. Conditions, however, vary from place to place; and, in some cases, the volume of the variable volume chamber and the flowing conditions must be varied to suit the peculiar circumstances.

When the chamber has thus been filled and sealed, the free piston is dropped into the tubing 10, preferably with the packing element on the bottom and the chamber on the top. The chamber is thus in better contact with the heat exchanger at the surface and is insulated from the hot well fluids. When the liquid in the chamber is cooled, the volume decreases, and the packer 21 is elongated, decreasing its external diameter. As the diameter becomes smaller than the internal diameter of the tubing, the free piston falls relatively freely through the tubing until its temperature is increased suificiently to expand the packer or until it strikes the bottom stop. At the lower end of the tubing, the free piston strikes the liquid before it reaches the bottom stop in the tubing, cushioning the fall of the plunger. In some cases, however, it may be desirable to attach some means, such as a rubber bumper, a spring, or the like, to cylinder head 43 or to the bottom stop, further to cushion the fall and prevent damage to the free piston. The free piston then rests on the bottom stop within the tubing as the tubing is filled with liquid. The temperature of the liquid in chamber 44 commences to rise due to contact with the hot well fluids. As this liquid temperature rises, the liquid expands, driving piston 41 up, compressing packer 21 longitudinally, and expanding the packer diametrically. When the temperature of the liquid has caused it to expand the diameter of the packer to approximately the internal diameter of the tubing, the packer seals the tubing and the free piston is raised to the surface by the well fluids.

While the operation of the free piston has been described in connection with a flowing well, it will be apparent that the operation is substantially the same when the thermally-sensitive free piston is used in a gas-lift well where the gas/oil ratio is always high enough to provide sufiicient cooling at the wellhead apparatus. While the gas/oil ratio is desirably high as in the case of gas-lift wells, most flowing wells have been found to have a gas/ oil ratio sufficiently high to provide adequate gas for cooling the thermally-sensitive free piston. I have found that in some cases the piston is lifted by oil containing no free gas but that by reducing the pressure on the oil as by passing the oil through the orifice, cold solution gas is evolved which provides cooling sufficient to actuate the control mechanism in the free piston.

As an example of the operation of this apparatus, a temperature-sensitive free piston was installed in a flowing well in Louisiana producing 84 barrels of oil per day from 2,900 feet with no water and a gas/ oil ratio of about 300 cubic feet per barrel. The well has a temperature gradient of about l.4 per 100 feet with a temperature difference of about 15 F. between the top of the well and the bottom of the paraflin deposit which was located at 1,100 feet. The wellhead temperature of the well fluids without the cooling apparatus was 80 F. The tubing in this well was cleaned of parafiin and a cooling apparatus in accordance with the above description was installed on the upper end of the tubing. The tubing pressure was 250 p. s. i. and there was a 200 p. s. i. pressure drop across the inch choke. The atmospheric temperature varied from 77 F. to 97 F. during the day, and with the artificial cooling apparatus installed, the temperature of the oil in the cooling chamber varied from 70 to 72 F.-8 to 10 F. lower than the normal temperature. The temperature-sensitive element of the free piston was filled with methyl alcohol having a coeificient of thermal expansion of 0.000618 and was set to expand the packing element at F., the temperature of the well fluids at 1,100 feet-the bottom of the parafiin deposit. The free piston was then dropped into the 2 /2 inch tubing. Due to temperature lag in the instrument, the piston fell to about 1,300 feet before the packer expanded. It returned to the surface in about one hour. It can thus be seen that whereas the normal difference between the oil temperatures at the surface and at the bottom of the deposit was only 15 F., by providing artificial cooling this temperature difference was increased to a minimum of 23 F. which is more than percent of the normal differential.

This difference in temperature was adequate to control the operation of the packer expanding mechanism on the free piston so that it would reciprocate in the tubing regardless of the ambient temperature, variations in flowing conditions, etc. and provide assurance that the piston would be lifted all the way to the surface. While the complete temperature range was increased more than 150 percent, probably the most significant result of artificial surface cooling was the wide temperature range provided at the surface which permitted the temperature-sensitive actuating mechanism to be pre-set so that the piston is sure to rise all the way to the surface and will then fall automatically. That is, a sensitivity of 1 F. or less at the surface was changed to a range of from 8 to 10 F.- a multifold increase.

Manifestly, the construction as shown and described is capable of some modification, and such modification as may be construed to fall within the scope and meaning of the appended claims is also considered to be within the spirit and intent of the invention.

I claim:

1. In combination a well tubing extending into a well, a free piston having a temperature-sensitive means to control the expansion and contraction of a paraflin scraper mounted on said free piston, an extension on the upper end of said tubing to receive said free piston, and an artificial cooling means for said free piston surrounding said extension.

2. A wellhead apparatus for cooling a free piston reciprocating in a well tubing comprising a tube having an inside diameter substantially equal to the inside diameter of said well tubing, a cooling chamber surrounding said tube, a choke forming a restricted passage between said tube and said cooling chamber, said tube being closed above said choke to form a closed damping chamber to cushion said free piston, and an outlet in said cooling chamber spaced from said choke to permit well fluids expanded at said choke to flow through said cooling chamber around said tube and exhaust through said outlet.

3. A wellhead apparatus according to claim 2 wherein said choke is adjustable.

4. An apparatus for artificially cooling a free piston operating in a well tubing comprising a tube connected to the upper end of said well tubing and closed at the upper end to form a damping chamber for cushioning said free piston, said tube having an inside diameter substantially equal to the inside diameter of said well tubing, an outside casing surrounding said tube and forming with said tube an annular cooling chamber, a restricted fluid passage through said tube entering said cooling chamber near the lower end, and a fluid outlet in said casing spaced from said restricted fluid passage to cause fluid circulation and cooling within said cooling chamber, said chamber and said tube extending above said passage a sufiicientdistance to provide space for said free piston in said tube and within said chamber above said passage.

5. An apparatus according to claim 4 wherein a needle valve is afiixed to said casing and adapted to throttle the flow of fluids through said restricted fluid passage.

6. In combination a well tubing extending into a Well, a free piston having a temperature-sensitive means to control the expansion and contraction of a parafiin scraper mounted on said free piston, an extension on the upper end of said tubing to receive said free piston, and an artificial cooling means for said free piston surrounding said extension, said artificial cooling means comprising a cooling chamber surrounding said extension, an outlet to said cooling chamber and a restricted passage between said extension and said cooling chamber, and said chamber. and said extension extending above said passage a suflicient distance to provide space for said free piston in said extension and within said chamber above said. passage whereby the volatile fluids produced by said Well are expanded and passed in indirect heat exchange, with said free piston.

References Cited in the file of this patent- UNITED STATES PATENTS 

